forbid to set current_block to Bad
forbid to set current_block to Bad

This was used as a hack to mark unreachable code by some people, but it
is prefered to simply use a new Block without predecessors, or set NULL
if you have to.

jonashaag/libfirm,jonashaag/libfirm,killbug2004/libfirm,MatzeB/libfirm,davidgiven/libfirm,jonashaag/libfirm,8l/libfirm,MatzeB/libfirm,libfirm/libfirm,jonashaag/libfirm,8l/libfirm,killbug2004/libfirm,8l/libfirm,libfirm/libfirm,MatzeB/libfirm,libfirm/libfirm,jonashaag/libfirm,killbug2004/libfirm,MatzeB/libfirm,davidgiven/libfirm,MatzeB/libfirm,MatzeB/libfirm,MatzeB/libfirm,davidgiven/libfirm,davidgiven/libfirm,killbug2004/libfirm,killbug2004/libfirm,8l/libfirm,jonashaag/libfirm,davidgiven/libfirm,libfirm/libfirm,libfirm/libfirm,killbug2004/libfirm,davidgiven/libfirm,killbug2004/libfirm,davidgiven/libfirm,8l/libfirm,8l/libfirm,8l/libfirm,jonashaag/libfirm